1. Technical Field
This invention relates to closed loop, electronic temperature control systems and, particularly to an electronic temperature control system for closely regulating the temperature of water discharged from a tankless water heater.
2. Discussion
Water heating devices are used for a variety of residential and industrial purposes. One particular type of water heating device, and probably the most common in use at the present time, is the conventional hot water tank system. With this system, water is pumped into a hot water holding tank and heated to a relatively high temperature, for example, 150.degree.-180.degree. Fahrenheit. If a particular use or application requires that the temperature of the water being used be less than the temperature at which the water tank is maintaining the water, then water from a cold water line must be added to the hot water discharged from the hot water tank to reduce the temperature of the heated water to the desired temperature level.
The conventional hot water tank system described above has numerous disadvantages. Probably the greatest disadvantage occurs when cold water is added to the heated water being discharged from the hot water tank to bring the overall temperature of the water down to the desired level. This results in a significant loss of energy in the form of heat dissipation from the hot water tank and additional heat dissipation to the environment through supply conduits between the hot water tank and the water outlet.
A less commonly used water heating device is a tankless, or "point-of-source", water heater. With the tankless water heater, incoming ground water passes through a component generally known as a heat exchanger and is instantaneously heated by heating elements within the heat exchanger until the temperature of the water leaving the heat exchanger matches a desired temperature set by a user of the system. With such systems the heat exchanger is typically heated by a large current flow which is regulated by an electronic control system. The electronic control system also typically includes a temperature selection device, such as a thermostat, by which the user of the system can select the desired temperature of the water being output from the heat exchanger.
A tankless water heater provides significant advantages over conventional hot water tank systems. One such advantage is the improvement in efficiency which the tankless water heater provides. Since energy in the form of heat is applied only when needed, i.e., only when hot water is desired, the energy loss which would otherwise occur in a conventional hot water tank system is avoided.
Although present day tankless water heating devices offer significant improvements over conventional hot water tank systems in terms of efficiency, applications exist where more improved regulation of the actual temperature of the output water is required. For example, the temperature of water used in photo-processing applications must be regulated closely to within a couple of degrees Fahrenheit of a set temperature. Such precise temperature control would require an electronic control system capable of "tracking" very closely a desired temperature selected by the user. The temperature control of present tankless water heaters can be affected to an unacceptable degree by varying flow rates and other factors, making them unsuited for temperature-critical applications.
Another disadvantage of present day tankless heating devices is the stress produced on heating elements within the heat exchanger when the heater is initially powered on, and full power is instantly applied to the heating elements. Current spikes can also occur under certain circumstances when power is removed from the system controlling the heat exchanger, and full power is applied to the heat exchanger. Such an instantaneous application of full power, which may range up to about 75 amps or more, can produce significant stress on the heating elements thus shortening their lives and compromising the durability of the device. The current spikes generated can also damage other electrically driven devices and equipment such as transformers of the utility supplying the power.
It is thus principal object of the present invention to provide an electronic temperature control system capable of more closely and accurately regulating the temperature of water output from a tankless water heating device.
It is a further object of the present invention to provide an electronic temperature control system which is capable of applying power gradually to heating elements of a heat exchanger within the tankless water heater when the heater is initially powered on, thus eliminating large current spikes that could otherwise result if full power was applied instantaneously to the heating elements.
It is still another object of the present invention to provide an electronic temperature control system operable to quickly interrupt the current being supplied to the heating elements within the device when the device is powered down, thus eliminating current spikes which could otherwise result if power was gradually removed from the system.
It is still another object of the present invention to provide an electronic temperature control system operable to controllably modulate the current flow to the heating elements within the heating device so as to apply current to the heating elements only when needed.